In the past 20 to 30 years, a number of systems for tracking and digitizing the X, Y location of a writing implement relative to a writing surface have been disclosed. For example, a summary of the work in this field is discussed in the background section of U.S. Pat. No. 5,248,856, the entire patent of which is hereby incorporated by reference.
In the system disclosed in the '856 patent, a reflective code structure is provided around the circumference of a pen toward the writing tip. Two scanning laser beams are used to monitor continuously the X, Y location of the reflective structure by triangulation.
A principal problem with the system disclosed in the '856 patent and other prior systems is that the X, Y location of the writing implement may be tracked and digitized even when the operator has moved the pen away from the board between writing events. This causes an incongruity between what the writer actually writes, or intends to write, and the image which is reproducible from the digitized record.
Several types of writing interruption events may occur during a writing or drawing session. A first type of interruption event occurs when the operator of the implement completely removes the pen out of the range of X, Y scanner detection. This type of interruption event is relatively easy to determine because when it occurs there is an absence of interaction between the implement and the scanners. Even if the system does not have a mechanism for concluding that absence of interaction between the implement and the scanners indicates movement of the pen to a non-writing mode, it does not matter because when the pen is not interacting with the scanners, there can be no X, Y tracking.
The present invention, however, addresses a second type of writing interruption event which most commonly occurs, for example, when the operator is printing letters. When moving a pen between printed letters, the operator typically lifts the pen only a small distance, for example, a fraction of an inch, above the writing surface. During this type of writing interruption event, the implement generally continues to interact with the X, Y scanning system causing unwanted lines, commonly referred to as "tracers," in the digitally reproduced image.
Tracers are especially problematic when the system is used to track the writing of a series of discrete symbols for which the meaning may be lost or changed when the symbols are connected by tracers. For example, Chinese characters, such as the one shown in FIG. 7, may completely lose or alter its meaning when the components of the symbol or adjacent symbols are connected by tracers.
Accordingly, an important objective of the present invention is to provide a system for quantitatively tracking movement of an object along an axis perpendicular to a planar surface.
More particularly, an objective of the invention is to provide a system and method for determining whether a writing implement is in a write mode or a non-write mode relative to a writing surface.
Another object of the invention is to provide a system which is capable of discriminating between board-contacted and board-non-contacted movement of a writing implement.
Another object of the invention is to distinguish between a write mode and a non-write mode of a writing implement with minimal if any occurrence of false detection due to pen tilting, fluctuation in pen velocity or other writing style idiosyncracies.
Another object of the invention is to provide pen-up/pen-down detection without interfering with the effective simultaneous detection of coding information on the pen for indicating the character, for example, color, of the pen.